1. Field of the Invention
The invention relates generally to the determination of strength parameters of materials. In particular, the invention relates to the determination of the strength of brittle-fracture materials, such as glass, glass ceramics, or ceramics. The invention is used more preferably for the determination of the fracture strength of glass panes.
2. Description of Related Art
Brittle-fracture materials behave differently from ductile materials, for example, in regard to failure under a load. Ductile materials, in particular many metals, expand under a bending or tensile load up to the elastic limit and then break apart at a relatively well-defined load. The fracture of brittle-fracture materials, by contrast, does not occur at a strength limit in terms of a material characteristic value, but rather statistically with a probability that depends on the acting tensile stress. The parameters of the fracture probability distribution (for example, normal or Weibull distribution) depend primarily on the processing of the samples—but, in contrast to ductile materials, they depend only slightly on the material.
In order to acquire knowledge on the strength of brittle-fracture materials, it is necessary to carry out fracture tests. The parameters of a fracture strength distribution, among other things, can then be derived from these tests.
If a plate-shaped element made of brittle-fracture material is subjected to bending, an additional effect is that the strength is significantly determined by the fracture strength of the edges of the element. Fractures that thereby occur in a surface area are generally less common than fractures that originate from one of the edges of the element.
Methods are known, with which the edge strength of thin glass can be determined without any contact. The US 2014/0083198 A1 describes, for example, a method in which plates are heated along the edge locally and without any contact. This results in the creation of tensile stress at the plate edges. Measurements using this method are costly in terms of time, however, and are not suitable either for obtaining statistical strength data or for process monitoring. Moreover, they fundamentally fail in the case of glasses exhibiting low expansion (low thermal expansion coefficient), such as those utilized in electronic devices, for example.